If the International Thermonuclear Experimental Reactor (ITER) experiment succeeds, India could potentially benefit in several ways. However, the most immediate advantage would likely be the ability to build fusion reactors for power generation, which is Option 4.

Fusion energy has the potential to be a game-changer for the world`s energy needs because it is a virtually unlimited source of clean energy that produces no greenhouse gases or other harmful pollutants. Fusion reactions occur when atomic nuclei are combined at extremely high temperatures and pressures, releasing vast amounts of energy. The challenge has been to create a sustainable fusion reaction that generates more energy than it consumes, which is what the ITER project aims to achieve.

India is one of the partner countries in the ITER project and has made significant contributions to its development. If the experiment is successful, India could gain access to cutting-edge fusion technology and expertise, potentially allowing it to build its own fusion reactors for power generation. This would not only provide a new source of clean energy but also help to reduce India`s dependence on fossil fuels and address the growing energy demand in the country.

Options 1 and 3 are also relevant to India`s energy needs, but they are not as directly related to the ITER project. India has large reserves of thorium, which could potentially be used as a fuel in nuclear reactors, but this technology is still in the experimental stage and has not yet been proven to be commercially viable. Similarly, improving the efficiency of fission reactors would be beneficial, but it is not directly related to the success of the ITER project.

Option 2, attaining a global role in satellite navigation, is not directly related to the ITER project at all. India is already a significant player in the field of satellite navigation, with its own satellite-based navigation system, NAVIC. While India`s space program is impressive, it is not directly related to the ITER project or the development of fusion energy.

In conclusion, the most immediate advantage for India if the ITER experiment succeeds would be the ability to build fusion reactors for power generation, which could provide a new source of clean energy and help to address the growing energy demand in the country.